The present invention relates to a process for the production of ferrochromium having a carbon content of 0.02 to 10% from iron-containing chromium ore by heating a mixture of chromium ore, solid carbon-containing fuel and slag-forming constituents in a rotary kiln to form a reaction product, and subsequently melting ferrochromium from the reaction product which is removed from the rotary kiln and cooled down before the melting.
Ferrochromium is an alloy which contains or consists of 20 to 70% chromium, 0.02 to 10% carbon, and the rest iron as well as the usual impurities. Ferrochromium is formed through a melting reduction of iron-containing chromium ores, especially chromium iron ore, with coal according to the equation EQU FeCr.sub.2 O.sub.4 +4C=Fe+2Cr+4CO.
The melting-reduction is conducted either with a lumpy ore-coke mixture or with ore-pellets and coke or with pre-reduced ore-fine coke-pellets and coke, particularly in a submerged arc furnace, whereby alloys with different carbon content result. Ferrochromium is used for the production of chromium steels as a pre-alloy. The very often undesired high carbon content of the ferrochromium alloys can be reduced through refining of the alloys or through refining of the chromium steel produced from them. Chromium ores generally contain or consist of 20 to 50% Cr.sub.2 O.sub.3, 10 to 40% FeO and 10 to 70% gangue. It is difficult to even partially separate the gangue before melting the ore, so that a high portion of gangue in the known melt-reduction process must be separated as liquid slag from the produced ferrochromium alloys. Since during this process, considerable portions of Cr.sub.2 O.sub.3 are still present in the reduction material along with the high melting gangue of the ore, the resulting slags have a high melting point, and melting temperatures of over 1750.degree. C. must be used, despite the addition of flux, in order to largely reduce the chromium oxide out of the liquid slag and to hold the chromium loss at low slag viscosity as low as possible. The high temperatures required for the melt-reduction necessitate an undesirably high use of energy.
A process for the production of carbon-poor ferrochromium is known from German Auglegeschrift NO. 2,062,641, by which a mixture of chromium ore and lime is burned in a cylindrical rotary kiln at more than 900.degree. C., preferably at 1100.degree. C. Then, 30 to 60% of this mixture is melted in an electric furnace into a synthetic slag. To this slag, there is subsequently added 70 to 40% of the burned mixture, more than 80% of the theoretically required amount of silicochromium in the melted state, and up to 20% of the required silicochromium amount in the solid state. This process has the disadvantage that the silicon content of silicochromium must be used as a reduction agent (44% Si, 36.5% Cr), and the entire gangue of the chromium ore is melted in an arc furnace and in the reaction pans.
German Auslegeschrift No. 1,014,137 discloses a process for the melting of iron-poor ore in a cylindrical rotary kiln, in which the pulverized ore is mixed with fuel and is heated to temperatures from 1100.degree. to 1300.degree. C., wherein the ore is reduced to metallic iron and magnetic iron oxide compounds, and in which subsequently the magnetic compounds of the reaction product are separated from the gangue by magnetic separation. Neither German Auslegeschrift No. 2,062,641, nor German Auslegeschrift No. 1,014,137 teach how a separation of the gangue can be achieved before melting the ferrochromium without causing work stoppages in the rotary kiln and without requiring reduction in the melting furnace.